Graphene and other layered material such as boron nitride (BN) comprise two-dimensional sheets of atoms or ions bound together in which the effective thickness of the sheet is in the order of atomic or molecular thicknesses (usually less than 2 nm), which is far smaller than the lateral dimensions of the order of micrometers up to centimeters. These and other thin-film materials such as thin Si layers, InGaAs, copper-indium-gallium-arsenide layers, have numerous potential applications as the conductor, semiconductor or insulator in electronic, optoelectronic, optical, sensor and thermal devices, and for mechanical strength and toughness applications, for storage, or for surface-modifying applications including sensors.
A key processing challenge to the development of a number of these practical applications however is the lack of a suitable method to pattern and transfer these films from their growth substrate to the target substrate, in a way that is reliable, robust, and suitable for manufacturing without damaging the properties of the transferred film. This is particularly challenging because the thin films are often in the single or few atomic sheet form, and are hence very fragile.
X. S. Li et al (Science 2009, 324:1312; ECS Transactions, 2009, 19:41) described one method in which a copper (Cu) growth substrate is etched away to release graphene onto the target substrate that is brought into contact with floating graphene in the etchant solution. This method does not provide for any possibility to register the transferred sheet to the target substrate. It also greatly limits applications because the target substrate may not be compatible with the etchant solution, which may further be a source of contamination.
X. Liang et al (Nano Lett. 2007, 7:3840) described the use of a thermal-release (TR) glue layer based on a resinous material that acts as a room-temperature glue to the graphene sheet together with a hard stamp made of silicon, to extract multilayer graphene sheets from a HOPG crystal, and deposit onto another thermal-curable glue layer or hydrophilic silicon oxide surface. Because it uses a glue layer to attach the graphene to the silicon stamp, the transfer of graphene to the target substrate depends crucially on the relative strength of adhesion to the target vs. the TR glue at the release temperature. This requires typically a glue layer to also be present on the target or a high surface energy substrate (such as hydrophilic silicon oxide surfaces). This greatly limits the applicability of this method because the target surface may not generally have a glue layer or a strongly adhesive surface, particularly for electronic, semiconductor and insulator applications. Furthermore the glue layer may cause structural damage to the graphene and severely contaminate the surfaces of the transferred 2-d layered material. A similar method is disclosed by Bae S. et al (Nature Nanotech, 2010, 5:574; ACS Nano, 2011, 6:2096), which also employs the thermal release tape as the support and transfer material and the process is carried out via a roll-to-roll method.
L. Song et al (ACS Nano 2009, 3:1353) described the use of gold as an adhesion layer to the patterned graphene film, and a thermal-release (TR) tape as the pressure-sensitive adhesive to adhere to the gold film to lift the gold/graphene composite film off the substrate. The assembly is then contacted onto the desired target substrate, and heated to the release temperature of the TR tape to release the gold/graphene composite film onto the target substrate. The gold is then attached off by standard potassium iodide etchant. This approach relies on strong adhesion between the target substrate and the graphene to successfully compete with the TR tape for the graphene, and requires a glue layer or a high energy surface to be present at the desired target substrate. In addition, the use of gold (or other metal) as the adhesion layer to transfer graphene causes general incompatibility with substrates that cannot tolerate gold (or other metal) etchants.
Similarly, S. Unarunotai et al (Appl. Phys. Lett. 2009, 95:202101; ACS Nano, 2010, 4:5591) described the use of gold and other metal films as an adherent layer on graphene to adhere to a pressure-sensitive polyimide tape. The entire assembly is then peeled off the substrate and laid on a target substrate. The polyimide tape is then etched off in oxygen plasma and the gold film etched off in a gold etchant. Such a peeling method would cause severe deformation and fracturing of the graphene film as revealed by the intensity of the defect band in Raman spectroscopy.
J. D. Caldwell et al (ACS Nano, 2010, 4:1108) also employs a related competitive adhesion approach by exfoliating graphene film from SiC using a thermal release tape. This transfer method leaves behind small areas of graphene on SiC surface because of poor conformal contact with the thermal release tape. An improvement of this method is also described (Carbon Nanotubes, Graphene, and Associated Devices Hi, Vol. 7761 (Eds: D. Pribat, Y. H. Lee, M. Razeghi), Spie-Int Soc Optical Engineering, Bellingham 2010) in which a poly(methyl methacrylate) (PMMA) layer is introduced between the thermal release tape and graphene. However, this method still relies on achieving a stronger adhesion of graphene to the target substrate than the thermal release tape.
M. J. Allen et al (Adv. Mater. 2009, 21:2098) describe the use of a poly(dimethylsiloxane) (PDMS) stamp to pick up graphene oxide sheets and few-layer graphenes (FLG) respectively deposited on a first substrate, to transfer to a second substrate. The transferred film quality via such direct physical exfoliation is poor and unreliable as evident by the optical images in that report. The two key problems in this method are that (i) the PDMS stamp does not have high affinity for the graphene sheets, and (ii) the graphene sheets that do adhere to the PDMS stamp may not transfer off to the target substrate if the adhesion between the graphene and the target is not high enough.
X. S. Li et al (Nano Lett. 2009, 9:435), X. S. Li et al (Science, 2009, 324:1312) and A. Reina et al (The Journal of Physical Chemistry C, 2008, 112:17741; Nano Letters, 2009, 9:30) use a polymer film of poly(methylmethacrylate) (PMMA) to act as a “carrier” film for the graphene sheet when the growth substrate is etched off. The PMMA/graphene composite film is then laid onto the target substrate and PMMA dissolved in acetone to transfer the graphene film. The chief disadvantage of this approach is that “carrier” film is fragile, and prone to warping, stretching or bending and wrinkling that causes micro-cracking and mechanical damage of the graphene sheet. Furthermore the inherent stress that is frozen in during the formation of the carrier film also causes deformation of the film (warping) which damages (fractures) the graphene film during transfer, thereby degrading its quality and limiting its application. Such methods would also not allow patterning underlying patterns on the target substrate. If patterned films are desired, these would have to be separately defined, and the carrier film method does not provide any possibility of accurate patterning on the target substrate.
It is clear that the above methods depend on having a first adhesive layer that adheres to the graphene sheet as a stamp or a carrier sheet, and achieves transfer to the target substrate by making the target surface even more strongly adhesive such as through the use of a second adhesive layer. This imposes stringent requirements on the nature of the substrate. Further, the use of adhesives may not be compatible with most applications in electronics and semiconductors, and further, the methods do not provide allowance of patterning a substrate.
The methods of the state of the art typically damage the thin film sheets by stress/strain during transfer due to warping, stretching or bending of the carrier film, and also allow contamination by the adhesion (glue) layer that cannot be generally removed, because the transferred graphene or thin film sheets cannot be subjected to harsh cleaning processes without damage. Moreover, some of the transfer methods require the use of a final chemical etching and/or cleaning step to remove the first adhesion layer which further limits the general compatibility with a number of substrates and manufacturing processes. The use of thermal-release tapes also inevitably causes stretching deformation and fracture of the attached thin film, and is inherently incompatible with accurate placement of patterned films on the target substrate. In addition, none of these transfer processes permits the simultaneous patterning of the 2-d thin films.
There is therefore a need for an improved process to transfer thin films from one substrate to another.